Modeling Diurnal Variation of SOA Formation via Multiphase Reactions of Biogenic Hydrocarbons

Abstract

Abstract. The daytime oxidation of biogenic hydrocarbons is attributed to both OH radicals and O3, while nighttime chemistry is dominated by the reaction with O3 and NO3 radicals. Here, the diurnal pattern of Secondary Organic Aerosol (SOA) originating from biogenic hydrocarbons was intensively evaluated under varying environmental conditions (temperature, humidity, sunlight intensity, NOx levels, and seed conditions) by using the UNIfied Partitioning Aerosol phase Reaction (UNIPAR) model, which comprises multiphase gas-particle partitioning and in-particle chemistry. The oxidized products of three different hydrocarbons (isoprene, α-pinene, and β-caryophyllene) were predicted by using near explicit gas mechanisms for four different oxidation paths (OH, O3, NO3, and O(3P)) during day and night. The gas mechanisms implemented the Master Chemical Mechanism (MCM v3.3.1), the reactions that formed low volatility products via peroxy radical (RO2) autoxidation, and self- and cross-reactions of nitrate-origin RO2. In the model, oxygenated products were then classified into volatility-reactivity base lumping species, which were dynamically constructed under varying NOx levels and aging scales. To increase feasibility, the UNIPAR model that equipped mathematical equations for stoichiometric coefficients and physicochemical parameters of lumping species was integrated with the SAPRC gas mechanism. The predictability of the UNIPAR model was demonstrated by simulating chamber-generated SOA data under varying environments day and night. Overall, the SOA simulation decoupled to each oxidation path indicated that the nighttime isoprene SOA formation was dominated by the NO3-driven oxidation, regardless of NOx levels. However, the oxidation path to produce the nighttime α-pinene SOA gradually transited from the NO3-initiated reaction to ozonolysis as NOx levels decreased. For daytime SOA formation, both isoprene and α-pinene were dominated by the OH-radical initiated oxidation. The contribution of the O(3P) path to all biogenic SOA formation was negligible in daytime. Sunlight during daytime promotes the decomposition of oxidized products via photolysis and thus, reduces SOA yields. Nighttime α-pinene SOA yields were significantly higher than daytime SOA yields, although the nighttime α-pinene SOA yields gradually decreased with decreasing NOx levels. For isoprene, nighttime chemistry yielded higher SOA mass than daytime at the higher NOx level (isoprene/NOx > 5 ppbC/ppb). The daytime isoprene oxidation at the low NOx level formed epoxy-diols that significantly contributed SOA formation via heterogeneous chemistry. For isoprene and α-pinene, daytime SOA yields gradually increased with decreasing NOx levels. The daytime SOA produced more highly oxidized multifunctional products and thus, it was generally more sensitive to the aqueous reactions than the nighttime SOA. β-Caryophyllene, which rapidly oxidized and produced SOA with high yields, showed a relatively small variation in SOA yields from changes in environmental conditions (i.e., NOx levels, seed conditions, and diurnal pattern), and its SOA formation was mainly attributed to ozonolysis day and night. To mimic the nighttime α-pinene SOA formation under the polluted urban atmosphere, α-pinene SOA formation was simulated in the presence of gasoline fuel. The simulation suggested the growth of α-pinene SOA in the presence of gasoline fuel gas by the enhancement of the ozonolysis path under the excess amount of ozone, which is typical in urban air. We concluded that the oxidation of the biogenic hydrocarbon with O3 or NO3 radicals is a source to produce a sizable amount of nocturnal SOA, despite of the low emission at night.

Modeling the influence of chain length on secondary organic aerosol (SOA) formation via multiphase reactions of alkanes

“…No significant impact of acidic seed on SOA formation appeared under similar experimental conditions. For example, non-seeded SOA growth is similar to SA-seeded SOA for theC10 alkane (i.e., C10A vs. C10B and C10C vs. C10D, respectively), suggesting that most alkane oxidation products are inert and less polar compared to other oxygenated products from biogenic (Yu et al, 2021;Beardsley and Jang, 2016;Han and Jang, 2022a) and aromatic HCs (Im et al, 2014;Han and Jang, 2022a).…”

“…The UNIPAR model's ability to accurately simulate SOA formation from various aromatic HCs (Im et al, 2014;Han and Jang, 2022a), monoterpenes (Yu et al, 2021), and isoprene (Beardsley and Jang, 2016) has been previously demonstrated. In this study, the UNIPAR model was extended to simulate the formation of SOA from multiphase reactions of linear alkanes including gas (g), organic (org), and inorganic (inorg) phases.…”

Modeling of the Atmospheric Process of Cyanobacterial Toxins in Algal Aerosol

“…The vital source of NO 3 is the decomposition of N 2 O 5 via a thermodynamic equilibrium reaction to form NO 2 and NO 3 . In the presence of aqueous aerosol, N 2 O 5 reacts with water forming HNO 3 , which significantly reduces the concentration of NO 3 . Therefore, the contribution of NO 3 might be insignificant for the MC-LR decay in ambient air.…”

“…In the presence of aqueous aerosol, N 2 O 5 reacts with water forming HNO 3 , which significantly reduces the concentration of NO 3 . 57 Therefore, the contribution of NO 3 might be insignificant for the MC-LR decay in ambient air. The sulfur species, including SO 2 , H 2 S, and dimethyl sulfide, are emitted from anthropogenic fuel combustions and wetlands and produce sulfuric acid via homogeneous gas-phase reactions and heterogeneous aqueous reactions in the aerosol phase.…”